One of the most economical methods of obtaining salt (herein used to refer to sodium chloride) is to pump sea water into holding ponds from which water is removed from the salt through natural solar evaporation. While the salt obtained in this manner consists mostly of sodium chloride, substantial amounts of impurities, including magnesium ions, calcium ions, sulfate ions etc. are present, and it is generally necessary to purify solar salt, even for industrial purposes. One means of reducing the calcium and magnesium content of salt to a level where the solar salt is useful for industrial purposes is to wash the solar salt with saturated brine. Such a washing procedure is described in "A New Process for the Washing of Solar Salt", de Flers et al., Second Symposium on Salt, Northern Ohio Geological Society, Inc., 1966, pp. 188-194 (Vol. 2).
For food grade salt, i.e., that which maybe added to commercially processed foods, even greater purity is required, including further reduction in non-sodium, non-chloride ions, and in particular, food grade salt must be substantially free of insoluble particles. Current government regulations require that food grade sodium chloride have a sufficiently low insoluble content to give the salt a milk pad rating of 1, i.e., very clean salt. The milk pad test, which rates the cleanliness of salt as follows.
250 grams of a representative sample of salt are weighed, transfered into a 4000 ml beaker and dissolved in 3000 ml deionized water using a laboratory mixer at moderate agitation. The solution is filtered through a 11/4" diameter lintine filter disc (obtainable from Sediment Testing Supply Co., 1512 W. Jarvis, Chicago, Ill. 60604). The beaker is washed with deionized water and all of the insolubles are carefully transferred from the beaker into the funnel carrying the sediment disc. After all the solution has passed through, the filter disc is removed and compared with U.S. Department of Agriculture Sediment Standards for Milk and Milk products, 7 CFR 58.2728. Rating of the pad is made by matching the sample pad with those exhibited in one of the standards.
Removal of insolubles represents a major obstacle in producing food grade sodium chloride from solar evaporated salt. A major source of these insoluble impurities is clay particles that are present in the salt as relatively large particles, i.e., 1/2 mesh Tyler or larger, when crude salt is harvested from the solar ponds. In the initial washing operation, some of these clay particles are broken down into fine suspended particles. The insoluble particles are affixed to the salt crystals and, previously it was believed that the insolubles were bound within the crystal structure of the salt grains during the crystallization process. Thus, recrystallization, a relatively expensive procedure, had been considered necessary for final purification to obtain food grade sodium chloride from solar salt.
The discovery has been made that insolubles are not, in fact, generally bound within the sodium chloride crystals but are instead adhered to the surfaces of the salt crystals from where they can be removed by particular washing of the salt with clarified brine. The freed insolubles are either washed away with the brine and may then be removed from the salt by mechanical means, including air flow removal of fine particles, and produce an acceptable food grade salt that meets all governmental standards.
It is a general object of the invention to produce very high grade sodium chloride, including food grade sodium chloride, from partially purified solar salt without resorting to recrystalization. More specifically, it is an object to reduce the insolubles in the salt to give the salt a milk pad rating of 3 or better for certain industrial applications and a milk pad rating of 1 for food grade salt.